A gallium nitride-based semiconductor transistor (hereinafter, referred to as a GaN-based semiconductor transistor) has, for example, an active layer composed of a GaN-based semiconductor and a gate insulating film formed on the active layer. As the gate insulating film, for example, SiO2, Al2O3, SiNx, GaOx, or HfO2 is used.
In the GaN-based semiconductor transistor, it is known that when SiO2 is used for the gate insulating film, the Si of the SiO2 diffuses into the GaN. In contrast, Japanese Patent Application Laid-Open (JP-A) No. 2008-277640 describes a technique in which, for a gate insulating film, Al2O3 is used alone instead of SiO2.
The technique disclosed in JP-A No. 2008-277640 can prevent the Si of the SiO2 from diffusing into the GaN. However, a gate insulating film composed of Al2O3 has poorer film characteristics than a gate insulating film composed of SiO2. The poorer film characteristics mean, for example, that leaked electric current is high, breakdown voltage is low, and the like.